Mass-independently fractionated sulfur in Archean paleosols: A large reservoir of negative Δ<SUP>33</SUP>S anomaly on the early Earth

Results of multiple sulfur isotope analyses from five Archean paleosols show the widespread presence of mass-independently fractionated sulfur in the regolith developed on the pre-2.5 Ga Earth. Analysis of sulfur from a small set of diamictite samples gave similar results. All values of Delta S-33 are negative, indicating that the Archean surface environments preferentially retained atmospheric S from the SO42- pathway, which carried a negative Delta S-33 signal, whereas a portion of the S from the S-8 pathway, with a positive Delta S-33, was transferred to the oceans. The soil SO42- was then converted to sulfide by bacterial sulfate reduction with terrestrial organic matter in the weathering horizon acting as a reductant. Some S from the S-8 pathway also included, which resulted in a net soil Delta S-33 anomaly from atmospheric S in the range-0.3 to-0.6%, similar to values for pyrites from floodplain sediments. Excess S-8-derived S was carried to the oceans, resulting in the negative/positive dichotomy between terrestrial and marine systems. Pyrite that formed in paleosols and pyrite grains that formed in flood-plain deposits, both carrying the terrestrial Delta S-33 signal, were then recycled into detrital pyrite grains nowfound in sand-stones and conglomerates deposited before the rise of atmospheric oxygen. Therefore the Earth's early regolith constituted a reservoir of S with negative Delta S-33 values that could be important for balancing the predominantly positive Delta S-33 signature found in marine sediments. (C) 2013 Elsevier B. V. All rights reserved.